The invention relates to a clutch device for a vehicle, comprising                a mechanical spring device, which constantly attempts to force a friction plate in a first direction for engagement of the clutch,        a pneumatic actuator which has a cylinder and a piston, which is movable therein, and which divides the cylinder into a main chamber and an auxiliary chamber,        a release bearing with a rotating part, which is connected to the friction plate and a rotationally stationary part, which is connected to the piston, and        a compressed air source, which is arranged via a controllable valve device for communication with the main chamber for movement of the friction plate in a second, opposite direction for disengagement of the clutch.        
The prior art that forms the basis for the invention will now be described with reference to FIGS. 1–5.
Clutches of the above-mentioned type may in principle be of a pushing type as illustrated in FIG. 1 or of a pulling type as illustrated in FIG. 2, where a longitudinal direction, i.e. axial direction for the clutch extends between the left and the right edges of the page relative to the reader. For the sake of simplicity, corresponding components of the clutches are indicated by the same reference numerals.
Both clutch types comprise a clutch cover 1, which is connected to a driving flywheel 2 of an engine (not shown). A friction plate 3 is axially movable and rotationally fixed to a driven shaft (not shown), which may be connected to a gearbox of the vehicle. The friction plate 3 is influenced via a pressure plate 3′ by a diaphragm spring 4, which constantly attempts to force the friction plate 3 into abutment against the flywheel 2 for rotationally fixed connection of the flywheel 2 with the driven shaft, thus enabling a torque to be transmitted from the flywheel to the driven shaft.
The diaphragm spring has a radially external portion 6, a central portion 7 and a radially internal portion 5, which are engaged with a rotating bearing race 19a of a release bearing 19 (FIG. 4), which is connected to an actuator 10. For the pushing clutch the actuator 10 is arranged to exert a pushing force F against the release bearing 19, i.e. a force that is exerted towards the left in FIG. 1, and for the pulling clutch the actuator 10 is arranged to exert a pulling force against the release bearing 19, i.e. a force that is exerted towards the right in FIG. 2.
It can be seen from FIG. 1 that the radially external portion 6 of the diaphragm spring 4 for the pushing clutch abuts against the pressure plate 3′, and that the diaphragm spring is tiltably connected with the clutch cover 1 at its central portion 7 in the same way as a cup spring. The diaphragm spring 4 is hereby pre-tensioned in such a manner that it constantly attempts to move the friction plate into engagement with the flywheel 2.
If the release bearing 19 is moved a distance s, the diaphragm spring 4 is tilted in the clockwise direction about the central portion 7, as indicated by the dotted line, and in such a manner that the radially external portion is moved away from the flywheel 2 and the clutch can be disengaged.
It can be seen from FIG. 2 that the central portion 7 of the diaphragm spring 4 for the pulling clutch abuts against the pressure plate 3′, and that the diaphragm spring's radially external portion 6 abuts against the clutch cover in such a manner that the diaphragm spring can tilt about this external portion 6.
If the latter diaphragm spring 4 is influenced by a pulling force F, it is bent outwards a distance s away from the flywheel 2 and tilts in the anticlockwise direction about the external portion 6, with the result that central portion 7 is moved away from the flywheel 2 and the clutch is disengaged.
From these figures it can be seen that radially extending portions of the diaphragm spring for the pushing clutch can be compared to a two-armed lever, while corresponding portions of the diaphragm spring for the pulling clutch can be compared to a one-armed lever.
For an actuator that exerts the same force F against the release bearing, a force K is obtained against the friction plate for both types of clutch.
K=F×B/A, where B is the distance from the release bearing to the tilting location, and A is the distance from the tilting location to the location where the diaphragm spring abuts against the friction plate, on the understanding that this equation illustrates a connection between K and F in the case where the diaphragm spring comprises only mutually independent, e.g. sector-shaped levers. A simplification of this kind, however, illustrates the principle of the mode of operation.
However, since the ratio B/A for the pulling clutch is greater than for the pushing clutch, for a pulling clutch a greater torque can be transmitted from the flywheel 2 to the driven shaft than for a pushing clutch on the assumption that for both clutches the same force is needed against the release bearing in order to disengage the clutch.
For vehicles where large torques are transmitted between the flywheel and the driven shaft, it is therefore advantageous to employ a pulling clutch, whereby the actuator and the force that has to be exerted for operation of the clutch may be relatively small.
In addition to the diaphragm spring, a spring device that influences the friction plate may also comprise other springs (not shown), whose spring force the release bearing also has to overcome during a disengagement of the clutch. A typical spring characteristic for such a spring device, i.e. the force that is exerted by the spring against the release bearing, as a function of the release bearing's travel distance s, is indicated in FIG. 3. This characteristic may be called a first function. Since this force is equal to the oppositely directed force exerted by the release bearing, these forces will be referred to hereinafter as F.
As illustrated in FIG. 3 where this spring characteristic is indicated by thick solid line C1, the force F increases initially during a first distance s1 until it reaches a maximum value, whereupon the force F decreases slightly immediately before the release bearing reaches a position wherein the friction plate 3 no longer abuts against the flywheel. During this distance s1 the clutch is therefore engaged.
This portion s1 is succeeded by a second curve area s2 and a third curve area s3 where the clutch is disengaged, the derivative of the first function, i.e. the spring characteristic, being negative in the area s2 but it may once again be positive in the area s3. Over large portions of the second area s2 the derivative of the first function is numerically relatively large, i.e. for a small travel distance for the release bearing a great change is obtained in the force F.
If the actuator is a hydraulic actuator, good, stable control of the actuator can be obtained by means of a proportional valve during engagement of the clutch despite the large, negative derivative of the first function or the spring characteristic during an engagement of the clutch, since the hydraulic fluid is practically incompressible.
In the case of large vehicles such as lorries and busses, compressed air is used at present for the operation of actuators for other systems in the vehicle, e.g. the braking system, which is advantageous since there is no need amongst other things for any return line leading used fluid back to a reservoir.
The use of a pneumatic actuator and a proportional valve for operation of the clutch presents some difficulties, however, since the engagement of the clutch may be imprecise on account of the substantial, negative derivative of the first function in the second area s2.
Attempts have been made to eliminate this difficulty by means of an additional spring or linearising spring, which provides a more linear, total characteristic together with the diaphragm spring. This linearising spring, however, contributes towards an increase in the total force and must be adapted to suit each individual diaphragm spring's characteristic.
Moreover, the use of a pulling clutch for such large vehicles is advantageous, since the forces that have to be exerted by the clutch actuator via the release bearing may be relatively small.
From WO 94/13972 a pneumatic, pulling clutch actuator is known, where the auxiliary chamber is used only for complete displacement of the piston during assembly or disassembly of the clutch device, or more specifically during a connection of the clutch actuator with the diaphragm spring or release of the clutch actuator from the diaphragm spring. Apart from this application of the auxiliary chamber, it has no function and makes the clutch device complicated and expensive.
An actuator that can be constructed in this way is illustrated in FIG. 4. The actuator 10 comprises an annular cylinder 11 with a radially internal cylinder wall 12 and a radially external cylinder wall 13, which are securely interconnected at their first ends via a first, circular end wall 14. At the second ends of the cylinder walls, a second, circular end wall 15 is sealingly attached to the external cylinder wall. Between the end walls 14, 15 there is mounted in the cylinder 11 a piston 16 with a tubular piston rod 17, which extends sealingly and slidably between the internal cylinder wall 12 and the radially internal end of the second end wall 15. The piston rod is securely connected to an external bearing race 19b of a release bearing 19, whose internal bearing race 19a is arranged for connection with a diaphragm spring 4, which can rotate relative to the actuator 10. The actuator therefore has a through-going, central passage through which can be passed a driven shaft, which can be connected to the friction plate 3, e.g. via longitudinal teeth of the friction plate and the driven shaft respectively.
The piston 16 together with the second end wall 15 and the first end wall 14 define a first cylinder chamber or main chamber 20 and a second cylinder chamber or auxiliary chamber 21 respectively.
For operation of this actuator a valve device 30 may be employed, which is illustrated in FIG. 5, where an actuator 10 is also schematically illustrated.
The valve device 30 comprises a compressed air source 31, a first valve 32 via which compressed air can be supplied to the main chamber 20 for movement of the piston 15 in the direction of the arrow A. For removal of compressed air from the main chamber 20, a second valve 33 is provided. A sensor 22 is provided to establish the position of the piston 15 and thereby the release bearing 19 relative to the cylinder 11. The valves 32 and 33 are used for normal operation of the clutch while the vehicle is running.
In addition, the valve device comprises a third valve 34, via which compressed air can be introduced into the auxiliary chamber 21 for movement of the piston against the direction of the arrow A, thus enabling the actuator 10 to be connected to or disconnected from the diaphragm spring 4. Furthermore, a fourth valve 35 is provided for removal of compressed air from the auxiliary chamber 21. A pressure gauge 23 may also be provided for measuring the pressure of the air in the auxiliary chamber 21.